According to the description on multimode optical fibers in IEC (International Electrotechnical Commission) standard IEC 60793-2 for optical fiber products, A1 optical fibers are multimode optical fibers and can be further classified into A1a optical fibers, A1b optical fibers, and A1d optical fibers according to different geometric structures. A1a optical fibers are graded-index optical fibers of 50/125 μm, A1b optical fibers are graded-index optical fibers of 62.5/125 μm, and A1d optical fibers are graded-index optical fibers of 100/140 μm. A1a optical fibers are the most widely-used commercial multimode optical fibers among the three types of optical fibers, and can be further classified into A1a.1 optical fibers, A1a.2 optical fibers, and A1a.3 optical fibers with bandwidth performance in ascending order, which respectively correspond to OM2 cabled optical fibers, OM3 cabled optical fibers, and OM4 cabled optical fibers in the ISO/IEC standard.
With the advantage of a low system cost, multimode optical fibers have become a superior solution in short-distance high-speed transmission networks and have been widely applied in data center, business center, high-performance computing center, storage area network, etc. Application scenarios of multimode optical fibers are usually integration systems like a narrow cabinet, a wire distribution box, etc., and the optical fibers will have a very small bending radius. While conventional multimode optical fibers are undergoing a small-angle bending, high-order modes transmitting near the edge of the fiber core leak out easily, which will result in signal loss. While designing a refractive index profile of a bend-insensitive multimode optical fiber, leakage of high-order modes can be prevented by way of adding a low refractive-index area in the cladding of the optical fiber, so as to minimize the signal loss. The excellent bend-insensitive property of bend-insensitive multimode optical fibers enables them to be efficiently applied in the local area network of a data center.
Inter-modal dispersion existed in multimode optical fibers greatly limits the transmission distance thereof. In order to reduce inter-modal dispersion in optical fibers, the refractive index profile of a core of a multimode optical fiber needs to be designed to have a refractive index continuously and gradually decreasing from the center to the edge. Said profile is usually called “α profile”. In other words, said profile needs to satisfy the refractive index distribution of the following power-exponential function:
            n      2        ⁡          (      r      )        =                              n          1          2                ⁡                  [                      1            -                          2              ⁢                                                          ⁢                                                                    Δ                    0                                    ⁡                                      (                                          r                      a                                        )                                                  α                                              ]                    ⁢                          ⁢      r        <    a  wherein n1 stands for refractive index of an optical fiber axis, r stands for distance away from an axis, a stands for radius of an optical fiber core, α stands for distribution index, and Δ0 stands for refractive index of a fiber core center relative to a cladding.
The relative refractive index is represented by Δi:
            Δ      i        ⁢    %    =            [                        (                                    n              i              2                        -                          n              0              2                                )                          2          ⁢                                          ⁢                      n            i            2                              ]        ×    100    ⁢    %  wherein ni stands for refractive index of a position i away from a fiber core center, and n0 stands for minimum refractive index of a core of an optical fiber, which is normally refractive index of a cladding of an optical fiber.
Doping agents with a refractive index adjustment function and at a certain concentration (such as GeO2, F, B2O3, P2O5, TiO2, Al2O3, ZrO2, SnO2, etc.) are doped in SiO2 to realize refractive index distribution of a core of a multimode optical fiber, and multimode optical fibers designed thereby can support a high-speed transmission of hundreds of meters. For example, with a laser light source of 850 nm, a single OM4 multimode optical fiber can support a more-than-550 m transmission of Ethernet traffic at a speed of 10 Gb/s and a more-than-150 m transmission thereof at a speed of 40 Gb/s. However, with the rapid development of the network transmission speed and users' increasing demands for bandwidth, capacity of multimode optical fibers needs to be continuously increased. At present, bandwidth of a single OM4 multimode optical fiber has come near the upper limit of a multimode optical fiber. In a single-light-source transmission system with a speed of 100 Gb/s, 400 Gb/s, or an even higher speed, the transmission distance that can be supported by the OM4 multimode optical fiber is greatly reduced. Wavelength division multiplexing (WDM) technology is an effective means that can further increase capacity of multimode optical fibers so that the multimode optical fibers can be better adapted to a higher-speed transmission system. With the WDM technology, a single optical fiber can accommodate multiple data channels, and each increase of one wavelength can enhance transmission capacity of optical fibers. For example, four wavelengths of 25 Gb/s are integrated to be transmitted by one multimode optical fiber, which realizes a property of a single multimode optical fiber to support a more-than-150 m transmission at a speed of 100 Gb/s, i.e., capacity of a single multimode optical fiber is increased to four times of the original transmission capacity. Application of the WDM technology in a multimode optical fiber requires the optical fiber to be able to support high-performance transmission under multiple wavelength windows.
Multimode optical fibers can obtain high bandwidth performance by way of precisely controlling refractive index distribution of a core. The bandwidth performance here refers to overfilled-launch (OFL) bandwidth of optical fibers, measured by the measurement test defined by the FOTP-204 standard prescribed in TIA. It is shown by research that, when a refractive index profile of multimode optical fibers is fixed, multimode optical fibers usually show high bandwidth performance only at a specific wavelength window, and when the optical fiber application window moves to a longer or a shorter wavelength, the bandwidth performance will get worse obviously. The relation between bandwidth of conventional OM3/OM4 multimode optical fibers and wavelength thereof is shown in FIG. 1, and as can be seen, the bandwidth performance sharply worsens outside the window of 850 nm. Obviously, said multimode optical fibers can hardly satisfy the requirement for application of the WDM technology.
U.S. Pat. No. 7,336,877 discloses an optical fiber having a core with a multi-segmented refractive index distribution which can support 2 GHz-km data transmission of one or more wavelength windows in a waveband between 775 nm and 1100 nm. However, said optical fiber fails to meet the standards for OM4 optical fibers, is unable to be compatible with conventional multimode optical fibers, and does not possess bend-insensitive properties. US Patent No. 2010254653 discloses a multimode optical fiber having anaprofile, and bandwidth performance of the optical fiber at windows of 850 nm and 1300 nm is optimized by way of Ge—F co-doping. However, said multimode optical fiber cannot satisfy the requirement for application of the WDM technology and does not possess bend-insensitive properties.
Therefore, it is necessary to design a multimode optical fiber which can not only be compatible with existing OM3/OM4 multimode optical fibers, but also have low bandwidth-wavelength sensitivity, can satisfy the requirements for application of the WDM technology in a certain waveband range, and possess excellent bend-insensitive properties, so as to satisfy the market demand for an increasing capacity of multimode optical fibers.